Vacuum ultraviolet (VUV) spectroscopy is used to determine the valence-band structure and location of defect states within the bandgap of porous organosilicate (SiCOH) dielectrics both before and after VUV and UV irradiation. SiCOH dielectrics have bandgap energies of about 9 eV. In addition, positive charge is trapped by defect states located 1 eV above the top of the SiCOH valence-band edge. These defect states can be populated or depopulated with electrons during UV and VUV irradiation, respectively. This is verified by measuring the magnitude and polarity of the trapped charge after VUV irradiation using two techniques: (i) capacitance vs voltage characteristics obtained with a mercury probe and (ii) surface-potential measurements obtained with a Kelvin probe. Both techniques show that the defect states are uncharged when occupied with electrons and positively charged when depleted of electrons. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3435285] All rights reserved.